Field-effect transistor, single-electron transistor and sensor using the same

ABSTRACT

A sensor capable of detecting detection targets that are necessary to be detected with high sensitivity is provided. 
     It comprises a field-effect transistor  1 A having a substrate  2 , a source electrode  4  and a drain electrode  5  provided on said substrate  2 , and a channel  6  forming a current path between said source electrode  4  and said drain electrode  5;  
         wherein said field-effect transistor  1 A comprises:   an interaction-sensing gate  9  for immobilizing thereon a specific substance  10  that is capable of selectively interacting with the detection targets; and   a gate  7  applied a voltage thereto so as to detect the interaction by the change of the characteristic of said field-effect transistor  1 A.

TECHNICAL FIELD

The present invention relates to a field-effect transistor, asingle-electron transistor and a sensor using the same.

BACKGROUND ART

A field-effect transistor (FET) and a single-electron transistor (SET)are elements that convert voltage signals input in a gate into currentsignals output from either a source electrode or a drain electrode. Onplacing a voltage between the source electrode and the drain electrode,charged particles existing in the channel move between the sourceelectrode and the drain electrode along the direction of electric fieldand are output from either the source electrode or the drain electrodeas a current signal.

At this point, the strength of the output current signal is proportionalto the density of the charged particles. When a voltage is applied onthe gate that is placed at upward, sideward or downward position of thechannel with an insulator therebetween, the density of the chargedparticles existing in the channel is changed. With the aid of thisproperty, the current signal can be varied by changing the gate voltage.Hereinafter, a field-effect transistor and a single-electron transistorare both called simply a “transistor” when they are not necessary to bedistinguished from each.

The currently known chemicals-sensing elements (sensors) usingtransistors are those utilizing the above-mentioned principles oftransistors. As a specific example of sensors, the one described inPatent Document 1 can be mentioned. Patent Document 1 discloses a sensorwith construction that a substance which is capable of selectivelyreacting with detection targets is immobilized on the gate of thetransistor. A change in the surface charge of the gate, induced by thereaction of the detection targets and the substance immobilized on thegate, varies the electric potential of the gate, thereby the density ofthe charged particles existing in the channel being changed. This changeleads to the variation in the output signal from either the drainelectrode or the source electrode of the transistor. Then the detectionof a detection target can be made by reading that variation.

[Patent Document 1] Japanese Patent Laid-Open Publication (Kokai) No.Hei 10-260156

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

However, in order for a sensor like that to be used as an immune sensorusing antigen-antibody reaction, extremely high detection sensitivityshould be required. Therefore, it has not yet realized because of itstechnical limiting factors to the detection sensitivity.

The present invention has been made in view of such problems asmentioned above. The object of the invention is to provide a sensor thatmakes it possible to detect detection targets with high detectionsensitivity.

Means for Solving the Problem

The present inventors have found that a sensor for detecting detectiontargets using a transistor is able to detect the detection targets withhigh detection sensitivity, when the transistor comprises not only asource electrode, a drain electrode and a channel but also aninteraction-sensing gate for immobilizing thereon a specific substancethat is capable of selectively interacting with the detection targetsand a gate applied a voltage thereto so as to detect the interaction bythe change of the characteristic of the transistor, and achieved thepresent invention.

That is, a sensor of the present invention is a sensor for detectingdetection targets, comprising a field-effect transistor having asubstrate, a source electrode and a drain electrode provided on saidsubstrate, and a channel forming a current path between said sourceelectrode and said drain electrode; wherein said field-effect transistorcomprises: an interaction-sensing gate for immobilizing thereon aspecific substance that is capable of selectively interacting with thedetection targets; and a gate applied a voltage thereto so as to detectthe interaction by the change of the characteristic of said field-effecttransistor. With this sensor, the interaction can be detected in a statewhere the transfer characteristic of the transistor shows the highestlevel in sensitivity. Thereby, the sensor can be highly sensitive.

Another sensor of the present invention is a sensor for detectingdetection targets, comprising a single-electron transistor having asubstrate, a source electrode and a drain electrode provided on saidsubstrate, and a channel forming a current path between said sourceelectrode and said e drain electrode; wherein said single-electrontransistor comprises: an interaction-sensing gate for immobilizingthereon a specific substance that is capable of selectively interactingwith the detection targets; and a gate applied a voltage thereto so asto detect the interaction by the change of the characteristic of saidsingle-electron transistor. With this sensor, the interaction can bedetected in a state where the transfer characteristic of the transistorshows the highest level in sensitivity. Thereby, the sensor can behighly sensitive.

As one preferred feature, said channel is formed with a nano tubestructure. With this construction, the sensitivity of the sensor can beimproved even further.

As another preferred feature, said nano tube structure is selected fromthe group consisting of a carbon nano tube, a boron nitride nano tubeand a titania nano tube.

As still another preferred feature, the electric characteristic of saidnano tube structure has the property like semiconductors.

As a further preferred feature, defects are introduced in said nano tubestructure. With this construction, a quantum dot structure can be formedwithin the nano tube structure.

It is also preferred that the electric characteristic of said nano tubestructure has the property like metals.

It is also preferred that said interaction-sensing gate is the othergate than said gate. With this construction, the transistor can be madeup with simple construction.

It is also preferred that said other gate is any one of a top gateprovided on the right side of said substrate, a side gate provided onthe side of said channel on the surface of said substrate or a back gateprovided on the back side of said substrate. With this construction, thedetection can be operated easily.

It is also preferred that said channel is bridged between said sourceelectrode and said drain electrode in the state where said channel isapart from said substrate. With this construction, the permittivitybetween the interaction-sensing gate and the channel gets lowered, whichresults in smaller capacitance of the interaction-sensing gate. Thus,the detection can be performed with high sensitivity.

It is also preferred that said channel is provided between said sourceelectrode and said drain electrode in the state where said channel isbent at room temperature. With this construction, the risk of damage tothe channel, which may be caused by a temperature change, can bereduced.

It is also preferred that said substrate is an insulated substrate.

It is also preferred that said channel is covered with an insulator.

With this construction, the current within the transistor can flowsurely in the channel. Thereby, the detection can be performed steadily.

It is also preferred that a layer of low-permittivity insulatingmaterial is formed between said channel and said interaction-sensinggate. With this construction, electric charge variation caused by theinteraction occurred at the interaction-sensing gate can be transmittedto the channel more efficiently. Thereby, the sensitivity of the sensorcan be enhanced.

It is also preferred that a layer of high-permittivity insulatingmaterial is formed between said channel and said gate. With thisconstruction, the transfer characteristic of the transistor can bemodulated more efficiently using the gate voltage applied to the gate.Thereby, the sensitivity of the sensor can be enhanced.

It is also preferred that said specific substance is immobilized on saidinteraction-sensing gate.

A field-effect transistor of the present invention is a field-effecttransistor used for a sensor to detect detection targets and having asubstrate, a source electrode and a drain electrode provided on saidsubstrate, and a channel forming a current path between said sourceelectrode and said drain electrode; wherein said field-effect transistorcomprises: an interaction-sensing gate for immobilizing thereon aspecific substance that is capable of selectively interacting with thedetection targets; and a gate applied a voltage thereto so as to detectthe interaction by the change of the characteristic of said field-effecttransistor.

Another field-effect transistor of the present invention is afield-effect transistor having a substrate, a gate, a source electrodeand a drain electrode provided on said substrate, and a channel forminga current path between said source electrode and said drain electrode;wherein said channel is a nano tube structure bridged between saidsource electrode and said drain electrode in the state where said nanotube structure is apart from said substrate.

Still another field-effect transistor of the present invention is afield-effect transistor having a substrate, a gate, a source electrodeand a drain electrode provided on said substrate, and a channel forminga current path between said source electrode and said drain electrode;wherein said channel is formed with a nano tube structure, and said nanotube structure is provided between said source electrode and said drainelectrode in the state where said nano tube structure is bent at roomtemperature.

Still another field-effect transistor of the present invention is afield-effect transistor having a substrate, a gate, a source electrodeand a drain electrode provided on said substrate, and a channel forminga current path between said source electrode and said drain electrode;wherein said channel is a nano tube structure, and said substrate is aninsulated substrate.

Still another field-effect transistor of the present invention is afield-effect transistor having a substrate, a gate, a source electrodeand a drain electrode provided on said substrate, and a channel forminga current path between said source electrode and said drain electrode;wherein said channel is a nano tube structure covered with an insulator.

A single-electron transistor of the present invention is asingle-electron transistor used for a sensor to detect detection targetsand having a substrate, a source electrode and a drain electrodeprovided on said substrate, and a channel forming a current path betweensaid source electrode and said drain electrode; wherein saidsingle-electron transistor comprises: an interaction-sensing gate forimmobilizing thereon a specific substance that is capable of selectivelyinteracting with the detection targets; and a gate applied a voltagethereto so as to detect the interaction by the change of thecharacteristic of said single-electron transistor.

Another single-electron transistor of the present invention is asingle-electron transistor having a substrate, a gate, a sourceelectrode and a drain electrode provided on said substrate, and achannel forming a current path between said source electrode and saiddrain electrode; wherein said channel is a nano tube structure bridgedbetween said source electrode and said drain electrode in the statewhere said nano tube structure is apart from said substrate.

Still another single-electron transistor of the present invention is asingle-electron transistor having a substrate, a gate, a sourceelectrode and a drain electrode provided on said substrate, and achannel forming a current path between said source electrode and saiddrain electrode; wherein said channel is formed with a nano tubestructure, and said nano tube structure is provided between said sourceelectrode and said e drain electrode in the state where said nano tubestructure is bent at room temperature.

Still another single-electron transistor of the present invention is asingle-electron transistor having a substrate, a gate, a sourceelectrode and a drain electrode provided on said substrate, and achannel forming a current path between said source electrode and saiddrain electrode; wherein said channel is a nano tube structure, and saidsubstrate is an insulated substrate.

Still another single-electron transistor of the present invention is asingle-electron transistor having a substrate, a gate, a sourceelectrode and a drain electrode provided on said substrate, and achannel forming a current path between said source electrode and saiddrain electrode; wherein said channel is a nano tube structure coveredwith an insulator.

As one preferred feature, said nano tube structure is selected from thegroup consisting of a carbon nano tube, a boron nitride nano tube and atitania nano tube.

As another preferred feature, the electric characteristic of said nanotube structure provided with above-mentioned field-effect transistor hasthe property like semiconductors.

As still another preferred feature, defects are introduced in said nanotube structure provided with above-mentioned single-electron transistor.With this construction, quantum dot structure can be formed within thenano tube structure.

As still another preferred feature, the electric characteristic of saidnano tube structure provided with above-mentioned single-electrontransistor has the property like metals.

Advantageous Effects of the Invention

According to the sensor of the present invention, it is possible todetect detection targets with high detection sensitivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (a) and FIG. 1 (b) illustrate sensors according to a firstembodiment of the present invention. FIG. 1 (a) is a perspective viewand FIG. 1 (b) is a side view of the transistor, respectively.

FIG. 2 (a) and FIG. 2 (b) illustrate sensors according to a secondembodiment of the present invention. FIG. 2 (a) is a perspective viewand FIG. 2 (b) is a side view of the transistor, respectively.

FIG. 3 (a) and FIG. 3 (b) illustrate sensors according to a thirdembodiment of the present invention. FIG. 3 (a) is a perspective viewand FIG. 3 (b) is a side view of the transistor, respectively.

FIG. 4 (a) and FIG. 4 (b) illustrate sensors according to a fourthembodiment of the present invention. FIG. 4 (a) is a perspective viewand FIG. 4 (b) is a side view of the transistor, respectively.

FIG. 5 (a) and FIG. 5 (b) illustrate sensors according to a fifthembodiment of the present invention. FIG. 5 (a) is a perspective viewand FIG. 5 (b) is a side view of the transistor, respectively.

FIG. 6 (a) and FIG. 6 (b) illustrate sensors according to a sixthembodiment of the present invention. FIG. 6 (a) is a perspective viewand FIG. 6 (b) is a side view of the transistor, respectively.

FIG. 7 (a) to FIG. 7 (d) are figures illustrating an example ofproducting method of a transistor according to an embodiment of thepresent invention.

FIG. 8 is a figure illustrating an example of producting method of atransistor according to an embodiment of the present invention.

FIG. 9 is a figure illustrating an example of producting method of atransistor according to an embodiment of the present invention.

FIG. 10 (a) to FIG. 10 (c) are figures illustrating examples of thepresent invention.

FIG. 11 is a figure illustrating examples of the present invention.

FIG. 12 (a) to FIG. 12 (c) are figures illustrating examples of thepresent invention.

FIG. 13 is a figure illustrating examples of the present invention.

FIG. 14 is a figure illustrating examples of the present invention.

FIG. 15 is a figure illustrating examples of the present invention.

FIG. 16 is a figure illustrating examples of the present invention.

FIG. 17 is a graph illustrating a result of examples of the presentinvention.

EXPLANATION OF LETTERS OR NUMERALS

1A-1F transistor

2 substrate

3 law-permittivity layer

4 source electrode

5 drain electrode

6 channel

7 side gate

9 back gate (non voltage-applied and electrode-construction type member)

10 antibody

11 high-permittivity layer

12 back gate

13 insulating membrane

14 top gate (non voltage-applied and electrode-construction type member)

15 side gate (non voltage-applied and electrode-construction typemember)

16 photoresist (channel protecting layer)

17 catalyst

18 CVD (chemical vapor deposition) furnace

19 carbon nano tube

20 spacer layer (insulating layer)

21 insulator

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, embodiments of the present invention will bedescribed. The present invention is not restricted to the followingembodiments, but any modification can be made without departing from thescope of the present invention. In the following first to fifthembodiments, the word “transistor” is used to mean both the field-effecttransistor and the single-electron transistor without beingdistinguished each other. Also, in the following embodiments, componentsthat are substantially the same have the same reference letters as inthe other embodiments.

[First Embodiment]

FIG. 1 (a) and FIG. 1 (b) illustrate sensors according to a firstembodiment of the present invention. As shown in FIG. 1 (a), thesubstrate 2 of the transistor 1A is made of insulating material. On theupside (upper side in the Figs.) of the entire substrate, the siliconoxide layer of insulating and low-permittivity material (hereinafter,called “low-permittivity layer,” if necessary) 3 is formed. On the rightside of the low-permittivity layer 3, the source electrode 4 and thedrain electrode 5, formed of gold, are placed, between which the channel6 formed with a carbon nano tube is bridged.

The channel 6 is bridged between the source electrode 4 and the drainelectrode 5 as if it were a bridge that connects the source electrode 4and the drain electrode 5. Thereby it is apart from the low-permittivitylayer 3 and the channel 6 is fixed to the transistor 1A at its twoconnecting points with the source electrode 4 and the drain electrode 5.The other portion of the channel 6 is in a state of being suspended inthe air. Furthermore, as shown in FIG. 1 (b), the channel 6 is bridgedin a state of being bent at a predetermined angle. The channel 6 is bentat a predetermined angle to the extent that the compressive or tensilestress exerted upon the channel 6 can be absorbed, for example when thedistance between the source electrode 4 and the drain electrode 5 isvaried by the deformation of the substrate 2 by a temperature change.Usually, the channel 6 is set up to bent at a predetermined angle aroundthe temperatures at which the sensor is used. Thereby it is able toabsorb the stress caused by the temperature change in the neighborhoodof the temperatures at which the sensor is used. But, in thisembodiment, the channel 6 is set up to bent at a predetermined angle atroom temperature.

At the opposed position of the channel 6 on the low-permittivity layer3, the side gate 7, formed of gold, is placed. The side gate 7 is placedfor applying a gate voltage to the channel 6. The source electrode 4,the drain electrode 5 and the side gate 7 are connected to an externalpower source, not shown in Figs., and are set up to be placed with avoltage by the external power source. Moreover, the currents through thesource electrode 4, the drain electrode 5 and the side gate 7 and thevoltages applied to them are measured by a measuring instrument, notshown in Figs., respectively.

The transistor 1A of this embodiment further comprises the back gate(the other gate) 9, as an interaction-sensing gate, formed of gold onthe back side (lower side in the Figs.) of the entire substrate 2, orthe opposite side to the low-permittivity layer 3. A specific substance(an antibody in the present embodiment) 10 that is capable ofselectively interacting with the targets to be detected by the sensor ofthe present embodiment is immobilized on the back gate 9.

The back gate 9 is set up not to be applied any voltages from outside.

With the above-mentioned construction of the sensor of the presentembodiment, before or during the measurement, the most suitable gatevoltage that the transfer characteristic of the transistor 1A shows thehighest sensitivity will be tested to be found, by adjusting the gatevoltage applied to the side gate 7. When the most suitable gate voltageis found, it will be set as the gate voltage applied to the side gate 7.

After that, maintaining the gate voltage to be the most suitable gatevoltage or in the vicinity thereof, a sample containing targets to bedetected is made to interact with the specific substance. The vicinityof the most suitable gate voltage means the range such that the changeof the characteristic of the transistor is expected to reach thesufficient level on the detection of the detection targets. If thesample contains any detection targets, the electric potential of theback gate 9 will be changed by the interaction between the specificsubstance and the detection target, which leads to the change in thecharacteristic value of the transistor, such as the current value of thecurrent between the source electrode and the drain electrode, thethreshold voltage or the gradient of the drain voltage to the gatevoltage, or in the characteristic value peculiar to single-electrontransistors, such as the threshold of Coulomb Oscillation, the CoulombOscillation period, the threshold of Coulomb Diamond or the CoulombDiamond period. By detecting this change, the interaction of thedetection targets with the specific substance can be detected.Furthermore, this detection indicates the existence of the detectiontargets in the sample.

According to the sensor of the present embodiment, as mentioned above,the transistor 1A can be set, using the side gate 7, in such a statethat the transfer characteristic thereof shows the highest level insensitivity, or that the transconductance thereof reaches the highestlevel. Thus, the influence given to the density change of the chargedparticles in the channel 6 by the change in the gate voltage, caused bythe interaction between the specific substance and the detectiontargets, can be maximized. As a result, the interaction between thespecific substance and the detection targets can be measured as a largerange of change in the characteristic of the transistor 1A.Consequently, the sensor according to the present embodiment can be setto be highly sensitive by amplifying the change in the characteristic ofthe transistor caused by the interaction between the specific substanceand the detection targets.

In addition, because a carbon nano tube, a kind of a nano tubestructure, is used as the channel 6, the detection targets can bedetected with higher sensitivity. Generally, the limit of the detectionsensitivity of a sensor using a transistor relates to the capacitance ofthe gate of the transistor (hereinafter, called “gate capacitance,” ifnecessary). The smaller the gate capacitance is, the wider range of thechange in the gate electric potential, which indicates the change in thesurface electric charge of the gate, can be detected and thus thedetection sensitivity of the sensor can be improved. Because the gatecapacitance is proportional to the product of the length of the channelL and the width W, or L×W, it is efficient to make the channel to beminute in order to reduce the gate capacitance. For these reasons, witha nano tube structure of this embodiment, which is extremely minute,detection targets can be detected with extremely high sensitivity.

Also, because the back gate 9 is used as an interaction-sensing gate andantibody is immobilized on the back gate, high sensitivity detection canbe performed with a simple construction. More specifically, because theback gate 9 is placed on the back side of the substrate 1A, thedetection can be operated easily.

Also, because the channel 6 is bridged between the source electrode 4and the drain electrode 5 in a state of being bent, the risk of damageto the channel 6, induced by its deformation by temperature changes, canbe reduced.

Also, because the substrate 2 is an insulating substrate, theinteraction at the back gate 9 can be surely detected.

Also, because the low-permittivity layer 3 is formed between the channel6 and the back gate (interaction-sensing gate) 9, the change in thesurface charge caused by the interaction at the back gate 9 istransmitted more efficiently as a change of the electric charge densityin the channel 6 and make itself shown as a large change in thecharacteristic of the transistor 1A. Consequently, the sensitivity ofthe sensor of the present embodiment is able to be highly enhanced.

Also, the use of the sensor of the present embodiment makes it possibleto real-time measure, thereby the monitoring of the interaction betweenmaterials being realized.

Also, conventional detection equipment using immunity reaction or thelike based on the other principles of detection, using markers likeradio immuno assay and chemiluminescence immuno assay, has been quitesatisfactory in its detection sensitivity. However, it requiresspecialized facilities and equipment systems and the measurement usingthem should be taken at the inspection center or the inspection room inthe hospital by the medical engineer who are the specialists in thefield. Thus, doctors in private practice can not obtain the result ofdetection quickly, because they have to perform inspections by orderingwith outside inspection centers. In addition, because they needs longreaction time, few of them can be used in emergency inspections. Thereason of the above-mentioned problems is that complicated operationslike washing are required in the reaction process owing to the use ofmarkers in the present immune measurement.

Further, though non-marker method immune sensors based on variousprinciples such as surface plasmon resonance (SPR) have been developed,they have all been used as equipment for researches so not yet inpractical use, for the lack of sufficient sensitivity fit forclinical-trial use. Also this immune sensor has a problem that the wholeequipment is very upsized by the use of optical detection method.

However, the sensor of the present embodiment can achieve the advantagessuch as downsizing of the sensor, the rapidity of detection, the easewith operation, and so on.

[Second Embodiment]

FIG. 2 (a) and FIG. 2 (b) illustrate sensors according to a secondembodiment of the present invention. As shown in FIG. 2 (a), thetransistor 1B constructing the sensor of the second embodiment of thepresent invention has similar structure to the transistor 1A describedin the first embodiment.

The transistor 1B of the present embodiment further comprises thephotosensitive resin layer of insulating and high-permittivity material(hereinafter, called “high-permittivity layer,” if necessary) 11 isformed on the upside of the entire low-permittivity layer 3. Thehigh-permittivity layer 11 is formed to cover the whole channel 6, thelateral sides of the source electrode 4, the drain electrode 5 and theside gate 7 but not to cover the upsides (upper sides in Figs.) of thesource electrode 4, the drain electrode 5 and the side gate 7. Thehigh-permittivity layer 11 is shown by chain double-dashed line in FIG.2 (a) and FIG. 2 (b).

Because the sensor of the present embodiment is constructed as mentionedabove, similar to the sensor of the first embodiment, it can be set insuch a state that the transfer characteristic of the transistor 1B showsthe highest level in sensitivity. Then, this makes it possible tomeasure the interaction between the specific substance and the detectiontargets as a wide range of change in the characteristic of thetransistor 1B. Consequently, the sensor sensitivity of the presentembodiment can be improved.

The following advantageous effects can be taken similar to the firstembodiment. That is, because a carbon nano tube is used as a channel 6,the sensor can have higher sensitivity. Also, because the back gate 9 isused as an interaction-sensing gate, high sensitivity detection can beperformed with simple construction and by simple operation. Also,because the substrate 2 is an insulating substrate, the interactionbetween the detection targets and the specific substance can be surelydetected. Also, because the low-permittivity layer 3 is formed betweenthe channel 6 and the back gate 9, the change in the surface chargecaused by the interaction at the back gate 9 is transmitted to thechannel 6 more efficiently, thereby the sensor sensitivity of thepresent embodiment can be further improved. Also, because the channel 6is bent, the damage induced by the variation of length, caused bytemperature changes or the like, can be prevented. In this embodiment,though the high-permittivity layer 11 is filled around the channel 6,photosensitive resin (photoresist), which consists of thehigh-permittivity layer 11, is formed of material that is soft enough toallow deformation of the channel 6, thereby the damage can be preventedas mentioned above.

Further, in this embodiment, because the high-permittivity layer 11, alayer of high-permittivity and insulating material, is formed betweenthe channel 6 and the side gate 7, the transfer characteristic of thetransistor 1B can be more efficiently modulated by applying the gatevoltage of the side date 7. Consequently, the sensitivity of the sensoris able to be further improved.

Also, because the channel 6 is covered with the insulating andhigh-permittivity layer 11, leaking of the charged particles in thechannel 6 to the outside thereof and intrusion of the charged particlesto the channel 6 from outside of the channel 6, other than from thesource electrode or the drain electrode, are able to be prevented. Thisresults in that the interaction between the specific substance anddetection targets can be detected steadily.

Also, the use of the sensor of the present embodiment makes it possibleto real-time measure, thereby the monitoring of interaction betweenmaterials being realized. Further, it can be integrated easily, therebya phenomenon of interaction between materials happened simultaneously inmany places can be measured at a time.

Also, the sensor of the present embodiment can achieve the advantagessuch as downsizing of the sensor, the rapidity of detection, the easewith operation, and so on.

[Third Embodiment]

FIG. 3 (a) and FIG. 3 (b) illustrate sensors according to the thirdembodiment of the present invention. As shown in FIG. 3 (a), thetransistor 1C constructing the sensors according to this embodiment,similar to the first embodiment, comprises the substrate 2 fabricatedwith insulating material, the insulating and low-permittivity layer 3,the source electrode 4 and the drain electrode 5. The source electrode 4and the drain electrode 5 are formed with gold. The channel 6 formedwith a carbon nano tube is bridged between the source electrode 4 andthe drain electrode 5.

The back gate 12 for applying a gate voltage to the transistor 1C isformed on the back side of entire substrate 2. The back gate 12 isconnected to a power supply, not shown in Figs., which applies a voltageto the back gate 12. The voltage placed to the back gate 12 is able tobe measured with a measuring instrument, not shown in Figs.

The membrane fabricated with silicon oxide (insulating membrane) 13 isformed, on the right side of the low-permittivity layer 3, from themiddle of the channel 6 to the edge of the layer 3 in the depthdirection in Figs.

The channel 6 penetrates the insulating membrane 13 sideways. In otherwords, the portion around the middle of the channel 6 is covered withinsulating membrane 13.

On the upper side of the insulating membrane 13, the top gate 14 isformed of gold as an interaction-sensing gate. Thus, the top gate 14 isplaced on the low-permittivity layer 3 with the insulating membrane 13between them. The top gate 14 is constructed not to be applied with anyvoltage from outside. Further, the antibody 10 or the specific materialis immobilized on the right side of the top gate 14.

The insulator 21 is formed on the right side of the entirelow-permittivity layer 3. The insulator 21 is formed to cover theportions of the channel 6 uncovered with insulating membrane 13, thelateral sides of the source electrode 4, the drain electrode 5, theinsulating membrane 13 and the top gate 14, but not to cover the upsidesof the source electrode 4, the drain electrode 5 and the top gate 14.The insulator 21 is shown by chain double-dashed line in FIG. 3 (a) andFIG. 3 (b).

Because the sensor of the third embodiment of the present invention isconstructed as mentioned above, similar to the sensor of the firstembodiment, it can be set in such a state that the transfercharacteristic of the transistor 1C shows the highest level insensitivity. Then, this makes it possible to measure the interactionbetween the specific substance and the detection targets as a wide rangeof change in the characteristic of the transistor 1C. Consequently, thesensor sensitivity of the present embodiment can be improved.

Also, similar to the first embodiment, because a carbon nano tube isused for the channel 6, the sensor can have higher sensitivity. Also,because the substrate 2 is an insulating substrate, the interactionbetween the detection targets and the specific substance can be surelydetected.

Also, because the top gate 14 is used as an interaction-sensing gate,high sensitivity detection can be performed with simple construction andby simple operation.

Also, because the low-permittivity insulating membrane 13 is formedbetween the channel 6 and the top gate 14, the change in the surfacecharge caused by the interaction at the top gate 14 is transmitted tothe channel 6 more efficiently, thereby the sensor sensitivity of thepresent embodiment can be further improved.

Also, because the channel 6 is covered with the insulator 21, leaking ofthe charged particles in the channel 6 to the outside thereof andintrusion of the charged particles to the channel 6 from outside of thechannel 6, other than from the source electrode or the drain electrode,are able to be prevented. This results in that the interaction betweenthe specific substance and detection targets can be detected steadily.

Also, the use of the sensor of the present embodiment makes it possibleto measure real-time, thereby to be realized the monitoring ofinteraction between materials. Further, it can be integrated easily,thereby a phenomenon of interaction between materials happenedsimultaneously in many places can be measured at a time.

Also, the sensor of the present embodiment can achieve the advantagessuch as downsizing of the sensor, the rapidity of detection, the easewith operation, and so on.

[Fourth Embodiment]

FIG. 4 (a) and FIG. 4 (b) illustrate sensors according to a fourthembodiment of the present invention. As shown in FIG. 4 (a), thetransistor 1D constructing the sensors according to this embodiment,similar to the first embodiment, comprises the substrate 2 formed ofinsulating material, the insulating and low-permittivity layer 3, thesource electrode 4 and the drain electrode 5. The source electrode 4 andthe drain electrode 5 are formed of gold. The channel 6 formed with acarbon nano tube is bridged between the source electrode 4 and the drainelectrode 5. And the transistor 1D also comprises the side gate 7.

The transistor 1D of the present embodiment further comprises the sidegate 15 as an interaction-sensing gate on the opposite-side edge oflow-permittivity layer 3 to the side gate 7, on the right side of whichantibodies 10 are immobilized. The side gate 15 is constructed that insuch a state that any voltages are not applied from the outside.

As shown in FIG. 4 (b), the transistor 1D of the present embodiment alsocomprises the insulator 21 provided on the upside of the entirelow-permittivity layer 3. The insulator 21 is formed to cover the wholechannel 6, the lateral sides of the source electrode 4, the drainelectrode 5 and the side gate 7, 15 but not to cover the upsides (uppersides in Figs.) of the source electrode 4, the drain electrode 5 and theside gate 7, 15. The insulator 21 is shown by chain double-dashed linein FIG. 4 (a) and FIG. 4 (b).

Because the sensor of the present embodiment is constructed as mentionedabove, similar to the sensor of the first embodiment, it can be set insuch a state that the transfer characteristic of the transistor 1D showsthe highest level in sensitivity. Then, this makes it possible tomeasure the interaction between the specific substance and the detectiontargets as a wide range of change in the characteristic of thetransistor 1D. Consequently, the sensor sensitivity of the presentembodiment can be improved.

The following advantageous effects can be taken similar to the firstembodiment. That is, because a carbon nano tube is used as a channel 6,the sensor can have higher sensitivity. Also, because the side gate 15is used as an interaction-sensing gate, high sensitivity detection canbe performed with simple construction. Also, because the substrate 2 isan insulating substrate, the interaction between the detection targetsand the specific substance can be surely detected.

Also, because the channel 6 is covered with the insulating insulator 21,leaking of the charged particles in the channel 6 to the outside thereofand intrusion of the charged particles to the channel 6 from outside ofthe channel 6, other than from the source electrode 4 or the drainelectrode 5, are able to be prevented. This results in that theinteraction between the specific substance and detection targets can bedetected steadily.

Also, the use of the sensor of the present embodiment makes it possibleto measure real-time, thereby to be realized the monitoring ofinteraction between materials. Further, it can be integrated easily,thereby a phenomenon of interaction between materials happenedsimultaneously in many places can be measured at a time.

Also, the sensor of the present embodiment can achieve the advantagessuch as downsizing of the sensor, the rapidity of detection, the easewith operation, and so on.

[Fifth Embodiment]

FIG. 5 (a) and FIG. 5 (b) illustrate sensors according to the fifthembodiment of the present invention.

The transistor 1E of the fifth embodiment of the present invention, asshown in FIG. 5 (a) and FIG. 5 (b), has similar construction to thetransistor 1A of the first embodiment, other than that it does notcomprise the back gate 9 and the antibodies 10.

That is, as shown in FIG. 5 (a), the substrate 2 of the transistor 1A isfabricated with insulating material. On the upside (upper side in theFigs.) of the entire substrate 2, the silicon oxide layer(low-permittivity layer) 3 of insulating and low-permittivity materialis formed. On the right side of the low-permittivity layer 3, the sourceelectrode 4 and the drain electrode 5, formed with gold, are placed,between which the channel 6 formed with a carbon nano tube is bridged.

The channel 6 is bridged between the source electrode 4 and the drainelectrode 5 as if it were a bridge that connects the source electrode 4and the drain electrode 5, thereby it is apart from the low-permittivitylayer 3. Thus, the channel 6 is fixed to the transistor 1A at its twoconnecting points with the source electrode 4 and the drain electrode 5.The other portion of the channel 6 is in a state of being suspended inthe air. Furthermore, as shown in FIG. 5 (b), the channel 6 is bridgedin a state of being bent at a predetermined angle. The channel 6 is bentat a predetermined angle to the extent that the compressive or tensilestress exerted upon the channel 6 can be absorbed, for example when thedistance between the source electrode 4 and the drain electrode 5 isvaried with deformation of the substrate 2 by a temperature change.Usually, the channel 6 is adjusted to bent at a predetermined anglearound the temperatures at which the sensor is used, thereby it is ableto absorb the stress caused by the temperature change in theneighborhood of the temperatures at which the sensor is used. But, inthis embodiment, the channel 6 is set up to bent at a predeterminedangle at room temperature.

At the opposed position of the channel 6 on the low-permittivity layer3, the side gate 7, formed with gold, is placed. The side gate 7 isplaced for applying a gate voltage to the channel 6. The sourceelectrode 4, the drain electrode 5 and the side gate 7 are connected toan external power source, not shown in Figs., and are set up to beplaced with a voltage by the external power source.

Because the transistor 1E of the present embodiment is constructed asmentioned above, owing to the fact that the channel 6 is bridged betweenthe source electrode 4 and the drain electrode 5 in a state of beingbent, the risk of damage to the channel 6, induced by its deformation bytemperature changes, can be reduced even with a temperature change whenit is used for detection or kept unused.

Also, because the substrate 2 of the present embodiment is an insulatingsubstrate, the permittivity of the substrate 2 can get lowered, whichleads to the reduction in the gate capacitance, thereby the sensitivityof the transistor 1E can be enhanced.

[Sixth Embodiment]

FIG. 6 (a) and FIG. 6 (b) illustrate sensors according to the sixthembodiment of the present invention.

The transistor 1F of the sixth embodiment of the present invention, asshown in FIG. 6 (a) and FIG. 6 (b), has similar construction to thetransistor 1B of the second embodiment, other than that it does notcomprise the back gate 9 and the antibodies 10.

That is, as shown in FIG. 6 (a), the transistor 1F constructing thesensor of the sixth embodiment of the present invention has similarstructure to the transistor 1E described in the fifth embodiment.

The transistor 1F of the present embodiment further comprises thephotosensitive resin layer of insulating and high-permittivity material(high-permittivity layer) 11 is formed on the upside of the entirelow-permittivity layer 3. The high-permittivity layer 11 is formed tocover the whole channel 6, the lateral sides of the source electrode 4,the drain electrode 5 and the side gate 7 but not to cover the upsides(upper sides in Figs.) of the source electrode 4, the drain electrode 5and the side gate 7. The high-permittivity layer 11 is shown by chaindouble-dashed line in FIG. 6 (a) and FIG. 6 (b).

The sensor of the present embodiment is constructed as above-mentioned.Thus, in this embodiment, because the high-permittivity layer 11, alayer of high-permittivity and insulating material, is formed betweenthe channel 6 and the side gate 7, the transfer characteristic of thetransistor 1E can be more efficiently modulated by applying the gatevoltage of the side date 7.

Also, because the channel 6 is covered with the insulating andhigh-permittivity layer 11, leaking of the charged particles in thechannel 6 to the outside thereof and intrusion of the charged particlesto the channel 6 from outside of the channel 6, other than from thesource electrode 4 or the drain electrode 5, are able to be prevented.This results in that the behavior of the transistor 1E can be stabled.

[Others]

Up to this point, though the present invention has been explained as theforms of the first to sixth embodiments, the present invention is notlimited to the above embodiments but also can be carried out withvarious modifications.

For example, above embodiments may be carried out in combinationarbitrarily.

For another example, the source electrode, drain electrode, gate,channel and interaction-sensing gate may be provided plurally.

For still another example, in the above embodiments, though theconstruction of the sensor is that the transistors are exposed, thetransistor may be installed inside of an appropriate housing orinstalled on the other equipment.

For still another example, though in the above embodiments the specificsubstances are immobilized on the sensors, the sensor may not beimmobilized with the specific substance thereon in the stage ofmanufacture or shipment, but the specific substance may be immobilizedby a user. That is, it is to be understood that the embodiments of thesensors of the present invention may include those that are notimmobilized with specific substances thereon.

For still another example, though the top gate, side gate and back gatehave been used as an interaction-sensing gate in the above embodiments,it is needless to say that any other gate, as well as any other membersthan gates, may be used as an interaction-sensing gate.

For still another example, though the channel 6 has been constructed tobe bent in the above embodiments, of course it may not be bent but bestraight.

For still another example, though the channel 6 has been bridged betweenthe source electrode 4 and the drain electrode 5 in the aboveembodiments, it may be provided being touched with the substrate 2 orthe low-permittivity layer 3, for example. As far as the channel 6 isbent, even in a state where it is touched with the substrate 2 or thelow-permittivity layer 3, the risk of damage to it caused by temperaturechanges can be reduced.

For still another example, a voltage can be applied on theinteraction-sensing gate.

[Components]

In the following, components in the above embodiments are describedparticularly.

As mentioned above, the word “transistor” in the above embodiments meanseither the field-effect transistor or the single-electron transistor.

A field-effect transistor and a single-electron transistor are the samein their basic structure, while the channels, which form current paths,of them are different. More specifically, the channel of thesingle-electron transistor has a quantum dot structure and the channelof the field-effect transistor does not. Thus, they can be distinguishedin the form of their construction by the existence of a quantum dotstructure.

<Substrate>

For a substrate (indicated by the numeral reference 2 in the aboveembodiments), those made of any materials may be used as far as they areinsulating substrates or insulated semiconductor substrates, without anyother restriction. But it is preferred to be insulating substrates orsubstrates with its surfaces covered with the material used for theinsulating substrates, when they are used in sensors. When an insulatingsubstrate is used, floating capacitance can be reduced because thepermittivity of an insulating substrate is lower compared to thesemiconductor substrate. Thereby, the detection sensitivity to detectthe interaction with can be improved when a back gate is used as theinteraction-sensing gate.

An insulating substrate is a substrate formed with an insulator, whichmeans an electric insulator as far as it is not specifically mentionedin the present specification. Examples of the insulator that forms theinsulating substrate are silicon oxide, silicon nitride, aluminum oxide,titanium oxide, calcium fluoride, acrylic resin, polyimide resin, Teflon(registered trademark), etc. These examples may used in any kinds ofcombination with any percentage of each.

A semiconductor substrate is a substrate formed with semiconductors.Examples of the semiconductor that forms the semiconductor substrate aresilicon, gallium arsenide, gallium nitride, zinc oxide, indiumphosphide, silicon carbide, etc. These examples may used in any kinds ofcombination with any percentage of each.

When an insulating membrane is formed on the semiconductor substrate inorder to insulate it, examples of the insulator that forms theinsulating membrane are the same as those used for the insulator thatforms the above-mentioned insulating substrate. In this case, thesemiconductor substrate can also work as a gate described below.

The shape of the substrate is not limited particularly. But it isusually manufactured in the form of a plate. The size of the substrateis not restricted, either. But it is preferred to be more than 100micrometers so as to maintain its mechanical strength.

<Source Electrode and Drain Electrode>

For a source electrode (indicated by the numeral reference 4 in theabove embodiments), there is no restrictions as far as it is anelectrode that can supply carriers of the above-mentioned transistors.For a drain electrode (indicated by the numeral reference 5 in the aboveembodiments), there is no restrictions either as far as it is anelectrode that can receive carriers of the above-mentioned transistors.

The source electrode as well as the drain electrode may be formed of anyelectric conductors. Examples thereof are gold, platinum, titanium,titanium carbide, tungsten, aluminum, molybdenum, chromium, tungstensilicide, tungsten nitride, polycrystal silicon, or the like. Theseexamples may used in any kinds of combination with any percentage ofeach.

<Gate>

For gates (indicated by the numeral reference 7, 12 in the aboveembodiments), any types of materials may be used as far as they are ableto control the density of the charged particles in the channel, withoutany other restriction. A gate is usually constructed to have a conductorinsulated from the channel and generally consisted of a conductor and aninsulator.

Examples of conductors forming gates are gold, platinum, titanium,titanium carbide, tungsten, tungsten silicide, tungsten nitride,aluminum, molybdenum, chromium, polycrystal silicon, etc. These examplesmay used in any kinds of combination with any percentage of each.

The location of the gate has no restrictions as far as the gate voltagecan be applied to the channel. For example, it may be installed as a topgate which is upward to the substrate, as a side gate on the samesurface as the channel of the substrate or as a back gate on the backside of the substrate.

In these kinds of gates, the top gate and the side gate may be formed onthe surface of the channel with an insulating membrane between them. Forthis insulating membrane, there is no restriction as far as it is aninsulating material, of which examples are inorganic materials likesilicon oxide, silicon nitride, aluminum oxide, titanium oxide, calciumfluoride, and polymer materials like acrylic resin, epoxy resin,polyimide, Teflon (registered trademark), etc.

<Interaction-Sensing Gate>

For an interaction-sensing gate (indicated by the numeral references 9,14 and 15 in the above embodiments), any types of members may be used asfar as they are able to immobilize specific substances thereon that arecapable of selectively interacting with the detection targets. Thus, theinteraction-sensing gate can be called an immobilizing member. Inanother aspect, the interaction-sensing gate can be called a nonvoltage-applied type immobilizing member because it is preferred not tobe applied with a voltage from outside. In still another aspect,conductors, semiconductors or even insulators may be used as aninteraction-sensing gate for example. But usually conductors are usedfor the interaction-sensing gate, as in the case of the source electrodeand the drain electrode. Thus, the interaction-sensing gate can becalled an electrode-construction type member. Further, to consider thatit is not applied with a voltage, too, it can also be called a nonvoltage-applied and electrode-construction type member. Examples ofconductors forming the interaction-sensing gate are gold, platinum,titanium, titanium carbide, tungsten, tungsten silicide, tungstennitride, aluminum, molybdenum, chromium, polycrystal silicon, etc. Theseexamples may used in any kinds of combination with any percentage ofeach.

Also, it is preferred that the gate without any gate voltage applicationin the transistor is used as an interaction-sensing gate. Morespecifically, the top gate, side gate or back gate can be selectedpreferably. Among the three of them, especially the top gate or backgate can be selected preferably. With the use of the top gate as aninteraction-sensing gate, the distance to the channel is generallysmaller compared to two other gates and thus the sensitivity of thesensor can be enhanced. If the back gate is used as aninteraction-sensing gate, the specific substance can be immobilized onthe interaction-sensing gate easily.

<Channel>

The channel (indicated by the numeral reference 6 in the aboveembodiments) may form a current path between the source electrode andthe drain electrode. Any known types of channels can be used, ifnecessary.

The channel is preferred to be passivated or protected with the cover ofinsulating materials. Any insulating materials can be used as theinsulating cover. Examples of them are polymer materials likephotoresist (photosensitive resin), acrylic resin, epoxy resin,polyimide, Teflon (registered trademark), and self-organizing membranelike aminopropylethoxysilane, lubricants like PER-fluoropolyether,Fonbrin (trade name), fullerene compound, or inorganic materials likesilicon oxide, fluosilicate glass, HSQ (Hydrogen Silsesquioxane), MSQ(Methyl Lisesquioxane), porous silica, silicon nitride, aluminum oxide,titanium oxide, calcium fluoride, daiamond thin film, etc. Theseexamples may used in any kinds of combination with any percentage ofeach.

It is also preferred that an insulating and low-permittivity layer(low-permittivity layer) is formed between the interaction-sensing gateand the channel. Further, the portion between the interaction-sensinggate and the channel has preferably low-permittivity characteristic allover (that is, over all of the layers between the interaction-sensinggate and the channel). Here, the phrase “low-permittivity” means thatthe relative dielectric constant thereof is smaller than 4.5.

Any insulating and low-permittivity materials can be used to constructthe low-permittivity layer, as mentioned above, without any otherrestrictions. Examples thereof are inorganic materials like silicondioxide, fluosilicate glass, HSQ (Hydrogen Silsesquioxane), MSQ (MethylLisesquioxane), porous silica, diamond thin film, and organic materialslike polyimide, Parylene-N, Palylene-F, polyimide fluoride, etc. Theseexamples may used in any kinds of combination with any percentage ofeach.

In fact, because the portion between the channel and theinteraction-sensing gate is insulating and low in permittivity, thechange of the surface charge on the interaction-sensing gate can betransmitted more efficiently in the form of the density change of thesurface change in the channel. Thereby, the above-mentioned interactioncan be detected as a wide range of change in the output characteristicof the transistor and thus the sensitivity of the sensor can be furtherimproved when the above-mentioned transistor is used as the sensor.

It is also preferred that an insulating and high-permittivity layer(high-permittivity layer) is formed between the gate for applying avoltage to the transistor and the channel. Further, the portion betweenthe gate and the channel has preferably high-permittivity characteristicall over (that is, over all of the layers between the gate and thechannel). Here, the phrase “high-permittivity” means that the relativedielectric constant thereof is bigger than 4.5.

Any insulating and high-permittivity materials can be used to constructthe high-permittivity layer, as mentioned above, without any otherrestrictions. Examples thereof are inorganic materials like siliconnitride, aluminum oxide, tantalum oxide, and polymer materials withhigh-permittivity characteristic. These examples may used in any kindsof combination with any percentage of each.

In fact, because the portion between the channel and the gate isinsulating and high in permittivity, the transfer characteristic of thetransistor can be modulated more efficiently with application of gatevoltage. Thereby, the sensitivity of the sensor can be further improvedwhen the above-mentioned transistor is used as the sensor.

Next, the channel of the field-effect transistor (hereinafter, called“FET channel”, when necessary) and the channel of the single-electrontransistor (hereinafter, called “SET channel”, when necessary) areexplained respectively. The field-effect transistor and thesingle-electron transistor can be distinguished by the channel of each,as mentioned earlier. In the above embodiments, a transistor should berecognized as a field-effect transistor when it has a FET channel, andas a single-electron transistor when it has a SET channel.

The FET channel may form a current path and any known channels can beused, if necessary. But the size of it is preferred to be minute.

An example of the minute channel is a nano tube structure. The nano tubestructure is a structure shaped like a tube, whose size is 0.4 to 50 nmin diameter of the cross section orthogonal to the longitudinaldirection. Here, a shape like a tube means a shape with the ratiobetween the longitudinal length and the longest orthogonal length amongall directions is in the range of 10 to 10000. It also includes shapessuch as a rod (almost circular in its cross section) or a ribbon (flatand almost square in its cross section).

A nano tube structure can be used as an electric charge transporter.Because it has a structure of one-dimensional quantum wire with severalnano meters in diameter, the gate capacitance gets remarkably loweredcompared to the field-effect transistors used in conventional sensors.Consequently, the change in the electric potential of the gate, causedby the interaction between the specific substance and the detectiontargets, gets extremely large. The change in the density of the chargedparticles existing in the channel gets extremely large, too. Thereby,the sensitivity of the detection for detecting targets is dramaticallyimproved.

Examples of the nano tube structure are a carbon nano tube (CNT), aboron nitride nano tube and a titania nano tube. With the conventionalfine processing technology of semiconductors, the detection sensitivityof the sensor has been restricted because of the difficulty in forming10-nanometer class channel. However, the use of these nano tubestructures makes it possible to form channels that are minuter than waspreviously possible.

A nano tube structure shows an electrical property like either asemiconductor or a metal, depending on its chirality. When it is used asan FET channel like a semiconductor, it is preferred that it has anelectrical property like a semiconductor.

The SET channel may also form a current path similarly to the FETchannel and any known channels can be used, if necessary. The size of itis also preferred to be minute. Further similarly to the FET channel, anano tube structure may be used as an SET channel and examples of thenano tube structure are a carbon nano tube (CNT), a boron nitride nanotube and a titania nano tube or the like.

However, the SET channel is different from the FET channel in the pointthat it has a quantum dot structure. Though any known materials withquantum dot structures can be used as SET channels, usually carbon nanotubes introduced with defects therein are used. More specifically,usually a carbon nano tube with 0.1 to 4 nanometer quantum dotstructures between the defects is used. It can be produced by a chemicalprocess such as heating a carbon nano tube without any defects in gasatmosphere like hydrogen, oxygen or argon, or boiling it in acidsolution

Thus, with these defects introduced in the nano tube structure, thequantum dot structures with regions sized several nanometers are formedbetween the defects. Thereby, the gate capacitance gets further lowered.A nano tube structure with quantum dot structures shows Coulomb Blockadephenomenon, which blocks the intrusion of electrons into the quantum dotstructure, and thus a single-electron transistor can be realized byusing that type of nano tube structure.

For example the gate capacitance of conventional silicon base MOSFET(metal-oxide semiconductor field-effect transistor) is about 10⁻¹⁵ F(farad), while the gate capacitance of the above-mentionedsingle-electron transistor with a nano tube structure introduced withthe defects is about 10⁻¹⁹ to 10⁻²⁰ F. That is, the gate capacitance ofa single-electron transistor decreases by a factor of about ten thousandto one hundred thousand, compared to the conventional silicon baseMOSFET.

As a result, the change in the gate capacitance, as well as the changein the density of the charged particles in the channel, can be extremelyenlarged through the use of a single-electron transistor having achannel formed with the above-mentioned nano tube structure, compared tothe conventional field-effect transistor without a nano tube structure.In addition, the detection sensitivity with which the detection targetsare detected can be highly improved.

The another point in which the SET channel is different from the FETchannel is that the nano tube structure used as the SET channel ispreferred to have the electric property like metals. Examples of methodsto check if the nano tube structure is like a metal or a semiconductorare the one with which the chirality of the carbon nano tube isdetermined by Raman spectroscopy or the other with which the electronicstate density of the carbon nano tube is measured using scanningtunneling microscope (STM) spectroscopy.

<Detection Target and Specific Substance>

The detection target is not restricted particularly but any materialscan be used. For specific substances (indicated by the numeral reference10 in the above embodiments), any types of materials may be used as faras they are capable of selectively interacting with the detectiontargets, without any other restriction. They may be, for example,enzyme, antibody, protein like lectin, peptide, hormone, nucleic acid,sugar, oligosaccharide, sugar chain like polysaccharide, lipid, lowmolecular compound, organic materials, inorganic materials, or fusion ofthese materials, and virus, cell, body tissue, or materials composingthem, etc.

The protein may be of its full length or may be a partial peptideincluding a binding activity portion. In addition, the protein with itsamino-acid sequence and function known, as well as unknown, may be used.Even synthesized peptide chain, proteins refined from an organism orproteins translated and refined from cDNA library or the like usingappropriate translation system can be used as the target molecules. Thesynthesized peptide chain may be bound with sugar chain to be aglycoprotein. In these examples, the synthesized protein with itsamino-acid sequence known or the protein translated and refined fromcDNA library or the like using appropriate process are preferably used.

The nucleic acid is not limited particularly, and DNA as well as RNA canbe used. The nucleic acid with its base sequence or function known, aswell as unknown, may be used. The nucleic acid with its function that ithas binding capacity in its protein and its base sequence known or thenucleic acid cut and isolated from genome library or the like usingrestricted enzyme and the like are preferably used.

The sugar chain with its sugar sequence or function known, as well asunknown, may be used. The sugar chain that is already isolated andanalized thus with its sugar sequence or function known is preferablyused.

The low-molecular compound is not limited particularly, as far as it iscapable of interaction. The low molecular with its function unknown, aswell as with its capability of bonding or reacting with proteins alreadyknown, can be used.

As mentioned above, lots of kinds of specific substances can beimmobilized on the interaction-sensing gate. The interaction-sensinggate immobilized specific materials thereon is suitably used for abiosensor used for detection of materials interacting with thefunctional material. The material, which interacts with the materialalready interacted with the specific substance, can be marked by enzyme,by materials showing electrochemical reaction or luminous reaction, orby polymer and particles with electric charge. These are well-knownmethod as labeling measurement in the field of DNA analysis usingimmunoassay or intercalator (cited references: Kazuhiro Imai,“bioluminescence and chemiluminescence,” 1989, Hirokawa Shoten or P.TIJSSEN “enzyme immunoassay biochemical experimental technique 11,”Tokyo Kagaku Dojin or Takenaka, Anal. Biochem., 218, 436 (1994) and thelike of many).

The “interaction” between the specific substance and the detectiontarget is not limited particularly, but usually means the action causedby the force acting between the molecules that are combined with atleast one type of combinations including covalent bond, hydrophobicbond, hydrogen bond, van der Waals binding or the combination made byelectrostatic force. However, the word “interaction” in the presentspecification should be interpreted most widely and in any meanings itshould not be interpreted limitatively. The covalent bond may includecoordinate bond and dipolar coupling. The combination made byelectrostatic force includes electrostatic repulsion, as well aselectrostatic binding. And also the interaction includes reactionsoccurred by the above-mentioned action, such as binding reaction,synthetic reaction or decomposition reaction.

Examples of the interactions are: binding and dissociation betweenantigen and antibody, binding and dissociation between protein receptorand ligand, binding and dissociation between adhesion molecule andobject molecule, binding and dissociation between enzyme and substrate,binding and dissociation between apoenzyme and coenzyme, binding anddissociation between nucleic acid and protein to be bound thereto,binding and dissociation between nucleic acids, binding and dissociationbetween proteins in communication system, binding and dissociationbetween glycoprotein and protein, binding and dissociation between sugarchain and protein, binding and dissociation between cells or bodytissues and protein, binding and dissociation between cells or bodytissues and low molecule compounds, interaction between ion andion-sensitive material or the like. However the interaction is notlimited to these types of actions, but also includes: immunoglobulin orthe derivation F(ab′)₂, Fab′, Fab, receptor or enzyme and the derivationthereof, nucleic acid, native and artificial peptide, artificialpolymer, carbohydrate, lipid, inorganic material or organic ligand,virus, cell, drug and the like.

Further, the other examples of the “interaction” between the specificsubstance immobilized on the interaction-sensing gate and the othermaterial can include the response, related to the functional materialimmobilized on the gate, not to any substances but to the change inoutside environments such as pH, ion, temperature, pressure or the like.

[Production Method of Transistor]

Next, an example of production method of the transistor, described inthe above embodiments, will be explained, using FIG. 7 (a) to FIG. 7(d), taking the case that a carbon nano tube is used as the channel forexample.

The transistor using a carbon nano tube is produced according to thefollowing procedure.

The carbon nano tube used in the transistor should be manufacturedcontrolling its position and orientation. Thus, it is usually producedusing patterend catalyst with a method like photolithography,controlling its position and orientation.

More specifically, a carbon nano tube is produced according to thefollowing steps.

(Step 1) Patterning Photoresist 16 on the Substrate 2, as Shown in FIG.7 (a)

The pattern corresponding to the position and the orientation where thecarbon nano tube will be formed is determined. Then, the photoresist 16is patterend according to the determined pattern on the substrate 2.

(Step 2) Evaporation Metal Catalyst 17, as Shown in FIG. 7 (b)

A metal that will be catalyst 17 is evaporated onto the patterendsubstrate 2. Examples of the metal to be catalyst 17 are transitionmetals like iron, nickel or cobalt, the alloys of them, etc.

(Step 3) Liftoff to Pattern with Catalyst 17, as Shown in FIG. 7 (c)

After the evaporation of the catalyst 17, lift-off is performed. Thephotoresist 16, as well as the catalyst 17 evaporated on the surface ofthe photoresist 16, are removed from the substrate 2 by the lift-off.Thereby, the pattern of the catalyst 17 is made according to the patternformed in step 1.

(Step 4) Feed the Gas Like Methane Gas or Alcohol Gas in CVD (ChemicalVapor Deposition) Furnace 18 at High Temperature and Forming Carbon NanoTube 19 Between the Catalysts 17, as Shown in FIG. 7 (d)

At high temperature, metallic catalyst 17 turns to be minute particlesof several nanometers in diameter, which is used as the core aroundwhich the carbon nano tube grows. Here, the high temperature usuallymeans from 300 to 1200° C.

After forming the carbon nano tube 19 following the steps 1 to 4, thesource electrode and the drain electrode are produced at the both edgeof the carbon nano tube 19. In this explanation, ohmic electrodes areformed as the source electrode and the drain electrode. At this point,the source electrode and the drain electrode can be installed at thetips of the carbon nano tube 19 or at the lateral side thereof. The heattreatment in the range of 300 to 1000° C. can be made when the sourceelectrode and the drain electrode are formed so as to get the improvedcondition of the electrical connections.

Then, the gate and the interaction-sensing gate are provided atappropriate positions, to finish the production of the transistor.

According to the above-mentioned method of production, the transistorcan be produced forming the carbon nano tube 19 under control of theposition and the orientation thereof. However, the probability offorming the carbon nano tube 19, between the catalyst metals is small byabove-mentioned method of production (about 10% in the test performed bythe present inventors). So, as shown in FIG. 8, the shape of thecatalysts 17 are made to be pointed sharply and an electric charge isapplied between the two catalysts while the carbon nano tube 19 isgrowing. This raises expectations that the carbon nano tube 19 growsbetween the sharp catalysts along the electrical flux line.

The reason why the carbon nano tube 19 grows when an electric charge isapplied between the catalysts 17 is not yet understood. However, thefollowing two reasons are guessed. One is that the carbon nano tube 19,starting to grow from the electrode (catalyst 17, in this example), haslarge polarization moment and thus it grows along the direction of theelectric field. The other is that the carbon ions decomposited at hightemperature form the carbon nano tube 19 along the electrical flux line.

Also it is probable that large Van der Waals force between the substrate2 and the carbon nano tube 19 may inhibit the growth of the carbon nanotube 19. The carbon nano tube 19 may be attracted to get close to thesubstrate 2 by Van der Waals force and the controlling of the directionthereof may be troubled. Thus, it is preferable to form the spacer layer20 of silicon oxide or the like between the catalyst 17 and thesubstrate 2, as shown in FIG. 9, which can make the carbon nano tube 19apart from the substrate 2 during the growth thereof.

According to the above-mentioned process, the field-effect transistorcan be produced.

Then, the single-electron transistor can be manufactured from thefield-effect transistor by chemical treatings such as heating inatmosphere gas of hydrogen, oxygen or argon, or boiling in the acidsolution so as to introduce the defects for forming quantum dotstructure therein.

[Immobilizing Method of Specific Substance to Interaction-Sensing Gate]

The method of immobilizing the specific substance to theinteraction-sensing gate is not limited particularly, as far as theimmobilization can be done with the method. The substance can be bindeddirectly to the interaction-sensing gate by physical adsorption, or by aflexible spacer, which has an anchor portion to bind the substance,placed at the interaction-sensing gate in advance, for example.

When a metal is used for the interaction-sensing gate, the flexiblespacer includes preferably alkylene with structural formula (CH₂)_(n) (nshows a natural number 1 to 30, preferably 2 to 30, more preferably 2 to15). The thiol group or the disulfide group, which forms an anchorportion suitable for adsorption of metals like gold, is preferably usedfor one end of the spacer molecule. One or more connecting portions thatcan bind the specific substance to be immobilized are preferablyincluded in the other end of the spacer molecule, which points to theother directions than to the interaction-sensing gate. The connectingportion may be various reactive functional group such as amino group,carboxyl group, hydroxyl group, suximide group, and hapten or chelatesuch as biotin, biotin derivative, digoxin, digoxigenin, fluorescein,fluorescein derivative, theophylline, for example.

In order for the specific substance to be immobilized to theinteraction-sensing gate, we can first bind conductive polymer,hydrophilic polymer, LB membrane or matrix to the interaction-sensinggate, directly or with a spacer therebetween, and then can connect,include or support one or more kinds of the specific substance on theconductive polymer, hydrophilic polymer, LB membrane or matrix. Andalso, we can first connect, include or support one or more kinds of thespecific substance on them and then can bind it to theinteraction-sensing gate.

The conductive polymer may be, for example, polypyrrole, polythiophene,polyaniline or the like. The hydrophilic polymer may be polymer withoutany electric charge such as dextran or polyethylene oxide, and may bepolymer with electric charge such as polyacrylic acid or carboxyl methyldextran. In the case of polymer with electric charge, with the use ofthe polymer with opposite electric charge to the specific substance tobe immobilized, the specific substance can be connected or supportedutilizing electric charge enrichment effect (cited document: farumashiapatent, Japanese Patent No. 2814639).

For detecting a specific ion, an ion-sensitive membrane corresponding tothe specific ion can be formed on the interaction-sensing gate. Inaddition, the enzyme immobilization membrane can be formed instead of ortogether with the ion-sensitive membrane. This results in that thedetection target can be detected by means of measuring the product ofenzyme, acted as a catalyst toward the detection target.

After immobilization of the specific substance to be immobilized,surface processing using inert molecules such as bovine serum albumin,polyethylene oxide or the like, covering with UF membrane in order toinhibit nonspecific reaction, or selecting permeable materials can bedone.

An insulating membrane can be selected as the interaction-sensing gate,other than metals. Examples of the materials used for the insulatingmembrane are inorganic materials such as silicon oxide, silicon nitride,aluminum oxide, titanium oxide, calcium fluoride, and polymer such asacrylic resin, epoxy resin, polyimide resin, Teflon (registeredtrademark), etc.

In order to measure ions like H⁺ or Na⁺, an ion-sensitive membrane canbe formed on the insulating membrane, if necessary, corresponding to therespective ions to be detected. In addition, the enzyme immobilizationmembrane can be formed instead of or together with the ion-sensitivemembrane. This results in that the detection target can be detected bymeans of measuring the product of enzyme, acted as a catalyst toward thedetection target (cited documents: Syuichi Suzuki “Biosensor” 1984Kodansha and Karube et al. “Development and practical application ofSensor,” vol. 30 No. 1 Bessatsu Kagaku Kogyo 1986).

[Application Areas]

The sensor of the present invention can be used in any appropriate area.Specific examples are in the following.

When it is used as a biosensor utilizing interactions, for example asensor for clinical examination on blood, urine or the like, pH,electrolyte, dissolved gas, organic materials, hormone, allergen,medication, antibiotic, activity of the enzymes, protein, peptide,mutagenic materials, cell of microorganism, blood cell, blood type,hemostasis, DNA analysis can be measured. In the point of the principleof measuring, examples of sensors to be considered are ion sensor,enzyme sensor, microorganism sensor, immune sensor, enzyme immunesensor, luminescent immune sensor, fungi count sensor, blood clottingelectrochemical sensing and electrochemical sensor using variouselectrochemical reactions. However it can contain all the principleswith which an electric signal can be available as the final output(cited documents: Syuichi Suzuki “Biosensor” Kodansha (1984) and Karubeet al. “Development and practical application of Sensor,” vol. 30 No. 1Bessatsu Kagaku Kogyo (1986)).

Furthermore, the sensor of the present invention can be used in themeasurement in vivo and in situ, for example an insertion-typemicrosensor mounted on a catheter, implantable microsensor orcapsule-mounted type microsensor using medical capsule (cited document:Karube et al. “Development and practical application of Sensor,” vol. 30No. 1 Bessatsu Kagaku Kogyo 1986).

EXAMPLES

In the following, examples of the present invention will be describedreferring to the Figs.

An field-effect transistor with a carbon nano tube as the channel wasproduced like the following.

[1. Production of the Sensor]

(Preparation of the Substrate)

After oxidating the surface of an n-type Si (100) substrate 2 by dippingit into an acid with the volume ratio between sulfuric acid and hydrogenperoxide is 4:1 for five minutes, it was rinsed out with water for fiveminutes. Then the oxicide layer was removed with acid with the volumeratio between hydrogen fluoride and pure water is 1:4, and finally itwas rinsed out to wash the surface of the Si substrate. Then the surfaceof the Si substrate 2 washed was thermally-oxidized with an oxidizationfurnace under the condition of 1100° C. temperature for 30 minutes withenzyme flow rate 3 L per minute. Consequently the insulating membrane 20was formed with about 100 nanometers-thickness of SiO₂.

(Formation of the Channel)

Next, the photoresist 16 was patterend by a photolithography method soas to form a carbon nano tube growth catalyst on the surface of theinsulating layer 20 {FIG. 10(a)}. First, the hexamethyldisilazane (HMDS)was spin coated on the insulating layer 20 under the condition of 500rpm for ten seconds, then 4000 rpm for 30 seconds. Then the photoresist(microposit “S1818” manufactured by Shipley Far East Ltd.) was spincoated under the same condition.

After the spin coat, the Si substrate 2 is baked on a hot plate under atthe temperature of 90° C. for one minute. Then, the Si substrate 2coated with photoresist 16 was dipped into monochlorobenzene and afterthat it was dried by nitrogen brow. Then it was baked in an oven at thetemperature of 85° C. for five minutes. After the baking, it wasexposured with a patterend enzyme using an aligner and then wasdeveloped for four minutes in developing agent (“AZ300MIF developer(2.38%)” manufactured by Clariant Co.). Then it was rinsed out withflowing water for three minutes and was dried by nitrogen brow.

The Si, Mo and Fe catalyst 17 is deposited on the Si substrate 2patterend with the photoresist 16 using an EB vacuum evaporation systemin order for the thicknesses of Si, Mo and Fe to be 100 Å, 100 Å and 30Å respectively, with deposition rate of 1 Å per second {FIG. 10(b)}.After the deposition, the lift-off was done in acetone which is beingboiled. Then it was washed by acetone, then by ethanol, and finally byflowing water, each for three minutes, and was dried by nitrogen brow{FIG. 10(c)}.

The Si substrate 2 patterend with the catalyst 17 was set in a CVDfurnace and blowed with ethanol, bubbled with Ar, with the rate of 750cc per minute and hydrogen with the rate of 500 cc per minute, at thetemperature of 900° C. for 20 minutes so as to make the carbon nano tube19 grow, which will be the channel (FIG. 11). In this process, heatingup and cooling down are done with Ar being blowed at the rate of 1000 ccper minute.

(Production of the Source, Drain and Side Electrodes)

After the growth of the carbon nano tube, the photoresist 16 waspatterend on the Si substrate 2 by a photolithography method, mentionedabove, so as to product the source electrode, drain electrode and sidegate {FIG. 12(a)}.

After the patterning, Ti and then Au are deposited on the Si substrate2, so as to form the source electrode 4, drain electrode 5 and side gateelectrode 7, using an EB vacuum evaporation system with the thicknessesof Ti and Au to be 300 Å and 3000 Å respectively and with the depositionrates of 0.5 Å and 5 Å per second respectively {FIG. 12(b)}. After thedeposition, the same as in the above-mentioned method, the lift-off wasdone in acetone that is being boiled. Then it was washed by acetone,then by ethanol, and finally by flowing water, each for three minutes,and was dried by nitrogen brow {FIG. 12(c)}.

After the patterning of the source electrode 4, drain electrode 5 andside gate electrode 7, HMDS is spin coated on the right side of the Sisubstrate 2, so as to protect the elements, under the condition of 500rpm for ten seconds, then 4000 rpm for 30 seconds. Then theabove-mentioned photoresist was spin coated under the same condition.Then the photoresist was hardened in an oven at the temperature 110° C.for 30 minutes so as to form a layer protecting the elements.

(Production of the Back Gate)

The SiO₂ layer 20 on the back side of the Si substrate 2 was removed bydry etching using RIE (reactive ion etching) device. Here, SF₆ was usedfor etchant and etching was done for six minutes in plasma with the RFpower of 100 W. After removing the SiO₂ layer 20 on the back side, Ptand then Au are deposited on the Si substrate 2, so as to form the backgate electrode (interaction-sensing gate) 9, using an EB vacuumevaporation system with the thicknesses of Pt and Au to be 300 Å and2000 Å respectively and with the deposition rates of 0.5 Å and 5 Å persecond respectively (FIG. 13).

(Formation of the Channel Layer)

Then the layer protecting the elements formed on the Si substrate 2 wasremoved by being washed in acetone being boiled, then by acetone,ethanol, and finally by flowing water each for three minutes.

Next, the photoresist was patterend, similarly to the photolithographymethod used for patterning the source electrode 4, drain electrode 5 andside gate electrode 7, onto the right side of the elements in the areawithout the source electrode 4, drain electrode 5 and side gateelectrode 7, so as to form the channel protecting layer 16 (FIG. 14).The schematic diagram of the carbon nano tube-field effect transistor(hereinafter, called “CNT-FET” when necessary) produced byabove-mentioned method is shown in FIG. 15.

[2. Characteristic Measurement Using the Sensor]

Using the CNT-FET produced, the characteristic before and afterimmobilization of the antibody was measured according to the followingprocess.

Fifty microliters of the mouse IgG antibody solution, which is dilutedby acetic acid buffer solution to have concentration of 100[microgram/milliliter], was dropped on the back gate 9, in order for theantibody to be immobilized, through the reaction in a wetting box at thehumidity of 90% for 15 minutes and the washing with pure water thesurface thereof. As the result of the immobilization, the specificsubstance, IgG antibody 10, was immobilized on the back gate electrode9, as shown in FIG. 16. The channel-protecting layer 16 is shown bychain double-dashed line in FIG. 16.

The measurement of electric characteristic of CNT-FET was conductedusing the semiconductor parameter analyzer 4156C manufactured by AgilentCo. The transfer characteristic (V_(SG)-I_(SD) characteristic), which isa kind of electric characteristic, was measured before and after theimmobilization of the antibody and then the two measured values werecompared. The result of the measurement is shown in FIG. 16. In themeasurement, at each stepped point of the side gate voltage V_(SG),which was sweeped from −40 to 40 V (stepped by 0.8 V), the current thatflows between the source electrode and the drain electrode (source draincurrent) I_(SD) A was measured while the source voltage V_(s) wasmaintained to 0 V and the drain voltage V_(D) was sweeped −1 to 1 V(stepped by 0.02 V). In FIG. 17, the graph lines of negative in thevalue of source drain current indicates the measured value while V_(SD)is −1.0 V and the graph lines of positive in the value of source draincurrent indicates the measured value while V_(SD) is +1.0 V.

To note the portion around 5 microamperes in the value of source draincurrent in FIG. 17, the side gate voltage after the immobilization ofthe antibody increased with +47 V from before the immobilization of theantibody, the change of which was remarkable. The measurement resultedin that the interaction occurred in the vicinity of the back gate can bedirectly measured because the transfer characteristic of CNT-FET isdramatically changed between before and after the immobilization of theantibody. This result indicates that the sensor of the present inventionhas extremely high sensitivity with which it detects chemicals and thusit is easily expected that the sensor is capable of detecting theinteraction between the detection target and the specific substance.

INDUSTRIAL APPLICABILITY

The present invention can be applied to wide range of analysis such aschemical analysis, physical analysis or biological analysis, and issuitably used for a medical sensor or a biosensor for example.

The invention claimed is:
 1. A sensor for detecting a detection target,the sensor comprising: a field-effect transistor having a substrate, asource electrode overlying the substrate and a drain electrode overlyingthe substrate, and a channel forming a current path between the sourceelectrode and the drain electrode; an insulator overlying the substrateand covering a portion of the channel; an interaction-sensing gateoverlying the insulator, the interaction-sensing gate having a specificsubstance that is capable of selectively interacting with the detectiontarget; a gate for applying a gate voltage to adjust a characteristic ofthe field-effect transistor; wherein the detection target changes thecharacteristic of the field-effect transistor when interacting with thespecific substance; wherein the interaction-sensing gate is not indirect contact with the source electrode and is not in direct contactwith the drain electrode; wherein the insulator covers only a middleportion of the channel; and wherein the specific substance is disposedon only a limited portion of the interaction sensing gate; wherein thechannel comprises a carbon nano tube, the carbon nano tube being bentbetween the source electrode and the drain electrode at roomtemperature.
 2. The sensor of claim 1, wherein the interaction-sensinggate is spaced from the substrate by the insulator.
 3. The sensor ofclaim 1, further comprising an insulator layer having a low-permittivityinsulating material.
 4. The sensor of claim 1, wherein the specificsubstance is disposed on a same side of the sensor as the sourceelectrode and drain electrode.
 5. The sensor of claim 1, wherein thecharacteristic is a current between the source electrode and the drainelectrode, a threshold voltage, or a gradient of the drawing voltage tothe gate voltage.
 6. A field-effect transistor used for a sensor todetect a detection target, the field-effect transistor comprising: asubstrate; a source electrode overlying the substrate and a drainelectrode overlying the substrate; a channel forming a current pathbetween the source electrode and the drain electrode; an insulatoroverlying the substrate and covering a portion of the channel; aninteraction-sensing gate overlying the insulator, theinteraction-sensing gate having a specific substance that is capable ofselectively interacting with the detection target; a gate for applying agate voltage to adjust a characteristic of the field-effect transistor;wherein the detection target changes the characteristic of thefield-effect transistor when interacting with the specific substance;wherein the interaction-sensing gate is not in direct contact with thesource electrode and is not in direct contact with the drain electrode;wherein the insulator covers only a middle portion of the channel; andwherein the specific substance is disposed on a limited portion of theinteraction sensing gate; wherein the channel comprises a carbon nanotube, the carbon nano tube being bent between the source electrode andthe drain electrode at room temperature.
 7. The field-effect transistorof claim 6, wherein the interaction-sensing gate is spaced from thesubstrate by the insulator.
 8. The field-effect transistor of claim 6,further comprising an insulator layer having a low-permittivityinsulating material.
 9. The field-effect transistor of claim 6, whereinthe specific substance is disposed on a same side of the sensor as thesource electrode and drain electrode.
 10. The field-effect transistor ofclaim 6, wherein the characteristic is a current between the sourceelectrode and the drain electrode, a threshold voltage, or a gradient ofthe drawing voltage to the gate voltage.
 11. A sensor for detecting adetection target, the sensor comprising: a single-electron transistorhaving a substrate, a source electrode overlying the substrate and adrain electrode overlying the substrate, and a channel forming a currentpath between the source electrode and the drain electrode; an insulatoroverlying the substrate and covering a portion of the channel; aninteraction-sensing gate overlying the insulator, theinteraction-sensing gate having a specific substance that is capable ofselectively interacting with the detection target; a gate for applying agate voltage to adjust a characteristic of the single-electrontransistor; wherein the detection target changes the characteristic ofthe single-electron transistor when interacting with the specificsubstance; wherein the interaction-sensing gate is not in direct contactwith the source electrode and is not in direct contact with the drainelectrode; wherein the insulator covers only a middle portion of thechannel; and wherein the specific substance is disposed on only alimited portion of the interaction sensing gate.
 12. The sensor of claim11, wherein the interaction-sensing gate is spaced from the substrate bythe insulator.
 13. The sensor of claim 11, wherein the channel comprisesa carbon nano tube, the carbon nano tube being bent between the sourceelectrode and the drain electrode at room temperature.
 14. The sensor ofclaim 11, further comprising an insulator layer having alow-permittivity insulating material.
 15. The sensor of claim 11,wherein the specific substance is disposed on a same side of the sensoras the source electrode and drain electrode.
 16. The sensor of claim 11,wherein the characteristic is a threshold of a Coulomb Oscillation, aCoulomb Oscillation period, a threshold of a Coulomb Diamond, or aCoulomb Diamond period.
 17. A single-electron transistor for a sensor todetect a detection target, the single-electron transistor comprising: asubstrate; a source electrode overlying the substrate and a drainelectrode overlying the substrate; a channel forming a current pathbetween the source electrode and the drain electrode; an insulatoroverlying the substrate and covering a portion of the channel; aninteraction-sensing gate overlying the insulator, theinteraction-sensing gate having a specific substance that is capable ofselectively interacting with the detection target; a gate for applying agate voltage to adjust a characteristic of the single-electrontransistor; wherein the detection target changes the characteristic ofthe single-electron transistor when interacting with the specificsubstance; wherein the interaction-sensing gate is not in direct contactwith the source electrode and is not in direct contact with the drainelectrode; wherein the insulator covers only a middle portion of thechannel; and wherein the specific substance is disposed on only alimited portion of the interaction sensing gate.
 18. The single-electrontransistor of claim 17, wherein the interaction-sensing gate is spacedfrom the substrate by the insulator.
 19. The single-electron transistorof claim 17, wherein the channel comprises a carbon nano tube, thecarbon nano tube being bent between the source electrode and the drainelectrode at room temperature.
 20. The single-electron transistor ofclaim 17, further comprising an insulator layer having alow-permittivity insulating material.
 21. The single-electron transistorof claim 17, wherein the specific substance is disposed on a same sideof the sensor as the source electrode and drain electrode.
 22. Thesingle-electron transistor of claim 17, wherein the characteristic is athreshold of a Coulomb Oscillation, a Coulomb Oscillation period, athreshold of a Coulomb Diamond, or a Coulomb Diamond period.